The microdevices manufactured by microelectromechanical (MEMS) technology are playing key roles in many areas. For instance, micromechanical gyroscopes have enabled several important control systems in transportation and commercial applications. Other microdevices such as pressure sensors, accelerometers, actuators and resonators fabricated by MEMS technology are also used in many areas. In addition, MEMS technology can be used to construct packaging using the same wafers from which the MEMS devices are made. For example, recesses can be formed in a wafer to create a cavity when mated to another wafer used to construct a MEMS device.
Some microdevices, such as micro gyroscopes and resonators contain a microstructure that needs to be maintained within a vacuum-sealed cavity. For these types of devices, there is a continuing need to improve the methods and techniques to hermetically seal the cavity to increase device lifetime. Pressure increases due to several sources can degrade device performance and reduce device lifetime for hermetically sealed microdevices.
One source that may cause pressure to increase in the cavity of a microdevice is inadequate sealing methods and techniques. For example, a microstructure has electrodes that need to be electrically connected to the outside environment in an appropriate way to meet further level packaging requirements, for instance, a surface mount capability. A suitable feedthrough design and hermetic seal method around the feedthroughs are needed to ensure a complete electrical connection and an appropriate cavity vacuum level over a device's lifetime.
For a wafer level hermetical seal, it has been known to electrically connect the electrodes of a microstructure through certain types of conductive vias formed in a wafer. This approach, however, has several disadvantages. For example, the formation of conductive vias in a wafer may result in mechanical damage to the microstructure mounted on the wafer. Additionally, vacuum degradation may occur over time due to microcracks or other defects in the conductive via. This is especially true if the conductive via extends into the vacuum-sealed cavity that houses the microstructure.
In addition, the different techniques used to manufacture recesses in a wafer to form a cavity cause their own problems that can contribute to leaks in the hermetic seal or other manufacturing problems. For example, recesses can be formed by chemical etching. However, this technique requires expensive equipment, uses dangerous acids, requires a long amount of time to accomplish, and requires various processing steps, methodologies, and precautions to prevent the etching of other surfaces on wafer that are desired to be left pristine. Moreover, it is very difficult to etch a wafer in preferential directions to obtain a desired cavity dimension.
Mechanical abrasion, such as sand blasting, can work beneficially to quickly remove material to form a recess without the manufacturing problems associated with etching. Moreover, mechanical abrasion can be accomplished at a reasonable cost. However, mechanical abrasion causes surface defects and damage problems. For example, mechanical abrasion results in a rough-textured, and micro-cracked surface. In addition, for transparent wafers, such as glass, mechanical abrasion results in an opaque surface, limiting an avenue to analyze the mechanical workings of an embedded device. In addition, vias made by mechanical abrasion can not be uniformly metalized due to the rough-textured surface and the rough-textured surface of the via walls has a high potential for failure due to crack propagation during subsequent high temperature solder processing.
Therefore, it is desirable to provide an improved technique to manufacture hermetically sealed microdevice packaging that overcomes most, if not all, of the preceding problems. It would also be beneficial if a technique could be provided to remove the damage in a glass cavity due to mechanical abrasion including restoring transparency, reducing stress cracking and improving surface finish.